fuel pump/relay
Thread Starter
Super Member
Joined: May 2003
Posts: 560
Likes: 1
From: Walnut, CA
MB 1990 300E
fuel pump/relay
Monday night my CEL light came on, so I took it into my mechanic to take a look. He said it was either the fuel pump or fuel pump relay. He cant really be sure which one it is. I looked on ebay and if i buy both with install is going to cost about 300. Is it possible to figure out which one is causing the problem? I would rather not replace both and spend 300 because im a little short on monies.
MBWorld Fanatic!
Joined: Apr 2005
Posts: 2,849
Likes: 3
From: Long Beach, Ca
lots of cars
If it's your fuel pump relay the car is going to shut off for no reason anywhere...Fuel pumps on these cars either work or they don't so the car would run with the fuel on the lines and then shut off and won't turn on again...If the fuel pump relay is making a buzzing noise when you turn the key it means it's bad...also turn the ignition and listen to the rear if you hear the fuel pump turned on....you have two fuel pumps in this car so make sure both are working..
Thread Starter
Super Member
Joined: May 2003
Posts: 560
Likes: 1
From: Walnut, CA
MB 1990 300E
hmm well the car drives fine. It doesn't shut off randomly or anything like that. The only difference I notice is really bad gas milage. Even worse than usual. Ill have to check out if i can hear any sound in the rear when i get back home. btw, where is the fuel pump and relay? lol 
Super Member
Joined: Jul 2005
Posts: 831
Likes: 1
From: Denver,CO USA
"88 300CE- First Year Production
I know this is an old thread but I have a few good questions here. A few weeks ago I was driving at a walmart and my car just completely shutoff while I was still in motion and I couldn't get the car to start at all. It turned over but wouldn't start, and at that point I knew that mean it just wasn't getting fuel to the engine. So I turn the key and don't hear the pumps on so eventually I orderes a fuel pump relay a like a charm it started. Just in case I ordered a used fuel pump on ebay as well. Here's what I need to get at since two years ago my car wouldn't start once after it was parked a while and my mechanic told me that a fuel pump and the relay needed to be replaced. At the time I had him just replace the bad one he was speaking of either the relay burned out the pump or vise versa, besides this and if thats true what makes the fuel pump relay go out like it did (2 pumps in just 2 years) Please somebody tell me more in a way it will make more since.
Super Member
Joined: Jul 2005
Posts: 831
Likes: 1
From: Denver,CO USA
"88 300CE- First Year Production
Okay guys I'll just try to test the pumps out my self. Do I just gotta test the fuel pressure to insure the correct flow is coming from the pumps. Or a way to test the pumps individually?
